1. Field of the Invention
The present invention relates to a mode-locked laser device, and particularly to a mode-locked laser device provided with a solid-state laser medium and a mode-locking element within a Fabry-Perot resonator.
2. Description of the Related Art
As a conventional technique for continuously generating optical short pulse train, a mode locking method is used in which a solid-state laser medium placed in a resonator is excited, for example, with a semiconductor laser, and phases of many lasing longitudinal modes are locked. The mode locking method includes, for example, an active method where an optical modulator is disposed in the resonator to apply loss modulation, and a passive method where a saturable absorber whose absorption coefficient changes nonlinearly is disposed in the resonator to achieve passive mode locking. A passive mode-locked laser device using a saturable absorber has been proposed, for example, in “Diode-pumped mode-locked Yb3+:Y2O3 ceramic laser”, OPTICS EXPRESS, Vol. 11, No. 22, pp. 2911-2916, Nov. 3, 2003, and International Patent Publication No. WO00/45480.
The mode-locked laser device typically uses a solid-state laser medium doped with a rare earth such as Yb (ytterbium) or Nd (neodymium). For example, a lasing threshold Pth of a laser device using a Yb-doped solid-state laser medium is expressed by formula (1) below, as described in Appl. Opt., Vol. 36, No. 9, pp. 1867-1874 (1997):
                                          P            th                    =                                                    π                ⁢                                                                  ⁢                                                      hv                    p                                    ⁡                                      (                                                                  ω                        L                        2                                            +                                              ω                        p                        2                                                              )                                                                              4                ⁢                                                      στη                    a                                    ⁡                                      (                                                                  f                        1                                            +                                              f                        2                                                              )                                                                        ⁢                          (                              Loss                +                T                +                                  2                  ⁢                                      N                    0                                    ⁢                                      f                    2                                    ⁢                  σ                                            )                                      ,                            (        1        )            wherein ωL represents the average beam radius (μm) of lasing light in the solid-state laser medium, ωp represents the average beam radius (μm) of excitation light in the solid-state laser medium, υp represents the frequency of excitation light, Loss represents the internal loss of the resonator, T represents the transmittance of the output mirror, σ represents the stimulated emission cross section (m2), τ represents the fluorescence lifetime (ms), ηa represents the excitation light absorption efficiency, No represents the amount of doped Yb ion, f1 represents the upper laser level local distribution probability, f2 represents the lower laser level local distribution probability, and h represents the Planck's constant.
As can be seen from formula (1), the lasing threshold can be lowered by reducing the beam radius (diameter) of lasing light and the beam radius (diameter) of excitation light within the solid-state laser medium. To reduce the beam radius (diameter) of lasing light within the solid-state laser medium, the resonator is typically designed so that a beam waist of the lasing light is formed within the solid-state laser medium. Further, in the passive mode-locked laser device using the saturable absorber, it is necessary to form another beam waist of the lasing light on the saturable absorber for achieving efficient mode locking. Since it is necessary to form two beam waists of the lasing light within the resonator, mode-locked laser devices disclosed, for example, in the above-mentioned International Patent Publication No. 00/45480 and “Diode-pumped mode-locked Yb3+:Y2O3 ceramic laser”, OPTICS EXPRESS, Vol. 11, No. 22, pp. 2911-2916, Nov. 3, 2003, employ three or more concave mirrors. This increases the number of parts forming the device, thereby making the device large and expensive, and poor in stability.
Further, in the lasing light obtained from the solid-state laser medium doped with Yb ion, three-level lasing is performed at the maximum peak in the fluorescence spectrum thereof, and therefore the lasing efficiency is significantly lowered by reabsorption loss due to electrons distributed at the lower laser level absorbing the lasing light. In order to avoid such reabsorption loss, it is necessary to fill the upper laser level with electrons by performing high-density excitation to minimize the reabsorption of the lasing light. In a case where a semiconductor laser is used as an excitation light source, due to a limitation in the power of commercially-available semiconductor lasers, it is necessary to increase laser density by reducing the beam diameter of excitation light within the solid-state laser medium and to increase the lasing efficiency by increasing overlap between the excitation light and the lasing light within the solid-state laser medium, in order to achieve high-density excitation. The reason for increasing the overlap between the excitation light and the lasing light within the solid-state laser medium is that, if the beam diameter of the excitation light is smaller than the beam diameter of the lasing light within the solid-state laser medium, a large reabsorption loss occurs at areas where no excitation light is present and the lasing efficiency decreases. In contrast, if the beam diameter of the excitation light is larger than the beam diameter of the lasing light within the solid-state laser medium, even areas where no lasing light is present, i.e., areas which do not contribute to lasing are excited and the lasing efficiency also decreases, and therefore sufficient high-density excitation may not be achieved.
On the other hand, the width of light with a wavelength of 940 to 980 nm emitted by a current commercially-available high-power semiconductor laser, which can excite the solid-state laser medium doped with Yb ion, is around 100 μm at the smallest. Therefore, in a Fabry-Perot resonator formed by a small number of parts, it is necessary to excite the solid-state laser medium via a resonator mirror. This increases the distance from the condensing lens to the solid-state laser medium, and necessitates a complicated excitation optical system to efficiently condense the light to have a small diameter.
In a case of a four-level system laser medium such as a solid-state laser medium doped with Nd ion, similarly to the above-described case of the solid-state laser medium doped with Yb ion, the lasing threshold can be lowered by reducing the beam diameters of the lasing light and the excitation light within the solid-state laser medium and increasing the overlap between the lasing light and the excitation light. Therefore, such device also has a complicated resonator structure and a complicated excitation optical system.
As described above, conventional mode-locked laser devices have multiple concave mirrors and thus have a complicated structure. They have poor output stability, and are large and expensive due to the large number of parts. In addition, they use a complicated excitation optical system to improve excitation efficiency, and this further increases the size of such devices.